1. Field of the Invention
The present invention discloses a unidirectional MOSFET and applications thereof.
2. Description of the Related Art
The N-channel MOSFET (NMOS) is often used as self-driven synchronous rectifiers to realize power converters with low cost and high efficiency. When used as a power switch, an NMOS has two operation modes: (1) cutoff mode: when vGS(t)<VGS(th), the channel cannot be formed; iD(t) cannot flow through the channel; (2) saturation mode: when vGS(t)≧VGS(th), the channel can be formed; iD(t) can flow from drain to source or from source to drain through the channel, where vGS(t) is the gate-source voltage; VGS(th) is the gate-source threshold voltage; and iD(t) is the drain current.
Due to the property of bidirectional conduction under the saturation mode, a synchronous rectifier realized with an NMOS usually suffers from a reverse current under light loads or a shoot-through current under heavy loads. The reverse current may degrade the converter efficiency and the shoot-through current may damage the synchronous rectifier. To illustrate the cause and effect of the shoot-through current, a flyback converter with a self-driven synchronous rectifier is taken as an example. If the flyback converter operates under the continuous conduction mode (CCM), and a cross conduction exists between the primary power switch and the secondary self-driven synchronous rectifier, the drain-source voltage divided by the small channel resistance will lead to a huge shoot-through current, which may damage the synchronous rectifier.
To solve the above problems, the present invention discloses a unidirectional MOSFET and its applications in synchronous rectifiers to detect the occurrence of a reverse current or a shoot-through current and fast turn off the channel of the MOSFET.